The present invention relates to the controlling of a vehicle's brakes to prevent the lock-up or skidding of a wheel against a surface. Skidding is defined as slippage in 100% of the tire/roadway interface. More specifically, the present invention relates to a brake control which utilizes the phase relationship between the brake pressure and the torque created from applying the brakes to prevent the skidding of the wheel.
There have been several attempts to control the skidding of a wheel. These attempts have been limited in that the skidding of a wheel must begin before the inducement of the anti-skid brake control. Therefore, there has not been established a means of controlling the brakes of a vehicle to prevent completely the skidding of a wheel.
One attempt to provide anti-skid brake control has been to utilize the speed of a wheel in relation to the speed of the ground. As stated above, this type of anti-skid brake control is not activated until the speed of the ground versus the speed of the wheel changes. Thus, the wheel begins to skid before the anti-skid brake control modulates the brakes. This method does prevent the actual lock-up of the wheel which causes a full skid, but does not prevent incipient skidding of the wheel on the surface.
Another attempt to provide an anti-skid control system for brakes utilizes the brake pressure and brake torque to vary the gain of the existing anti-skid system. This system uses an accelerometer to measure the brake torque combined with a pressure signal in a summing circuit to modify the control signal to limit the skidding of the wheel on the surface. Again, this system did not provide a true anti-skid control which prevents the skidding of the wheel.
Yet another attempt has been to utilize incremental changes in speed and torque to prevent the skidding of the wheel. This attempt has been to limit both the driving torque and the braking torque in order to control the skidding. The instantaneous frictional coefficient of the wheel is estimated from the wheel speed and torque signals to limit the skidding of the wheel.
Yet still another attempt has been to measure the brake pressure and torque developed by the brake pressure to limit the brake pressure thereby limiting the braking torque to prevent mechanical damage to the landing gear of an aircraft or of a vehicle. This system does not limit the skidding of the wheel but prevents excessive torque being applied to the wheel which may cause damage to the wheel.
Thus, it is desired to provide a control system for controlling the brake system of a vehicle that would prevent any skidding of the wheel on a surface. Such a system would prevent the skidding of the wheel on a surface having varying characteristics such as a mixture of ice or snow and dry spots. The system would have a quick response time in order to control the brakes under varying conditions to prevent any skidding of the wheel on the surface. Such a system would have to anticipate the beginning of a skid and prevent such skidding by modifying the brake control accordingly.